Field of the Invention
The present invention relates to an adaptive optics system and a control method of the same, a testing apparatus and a control method of the same, an information processing apparatus and a control method of the same, and a computer-readable storage medium and, more particularly, to an adaptive optics system for correcting a wavefront aberration which occurs in an object.
Description of the Related Art
Recently, an adaptive optics (to be referred to as “AO” hereinafter) technique which corrects up to high-order wavefront aberrations by using an active optical element has been put into practical use, and are applied to various fields. In this technique, the wavefront aberration of return light from an object, which is caused by, for example, the optical characteristics of the object itself or variations in measurement environment when the object is irradiated with illumination light, is successively measured by a wavefront sensor and corrected by a wavefront corrector. Examples of the wavefront corrector are a deformable mirror (to be referred to as a “DM” hereinafter), and a spatial light modulator (to be referred to as an “SLM” hereinafter). The AO was initially invented for the purpose of correcting the disturbance of the wavefront caused by the atmospheric fluctuation during astronomical observation, thereby improving the resolution. Recently, however, a testing system for the retina of an eye is particularly attracting attention as an application field having a large effect of introduction.
Known examples of the testing system to be used as an ophthalmologic instrument are a fundus camera, a scanning laser ophthalmoscope which acquires the retina as a two-dimensional image as a plane, and an optical coherence tomography which noninvasively acquires a tomographic image of the retina. In the following description, the scanning laser ophthalmoscope will be referred to as an “SLO”, and the optical coherence tomography will be referred to as an “OCT”. In the SLO and OCT, a deflector one-dimensionally or two-dimensionally scans irradiation with a light beam on the retina, and synchronously measures reflected light and backscattering light from the retina, thereby acquiring a two-dimensional image or three-dimensional image of the retina.
The spatial resolution (to be referred to as a “lateral resolution” hereinafter) of the acquired image in the plane direction (lateral direction) of the retina is basically determined by the diameter of a beam spot scanned on the retina: the smaller the beam spot diameter, the higher the lateral resolution of the acquired image. To decrease the beam spot diameter condensed on the retina, the diameter of a beam entering the eye need only be increased. However, the surface shape or refractive index of the cornea or crystal lens mainly having a refracting action in the eyeball is not even, and this characteristic of the eye optical system generates a high-order aberration on the wavefront of transmitted light. Even when a thick beam enters, therefore, a spot on the retina cannot condense to a desired diameter but widens. As a consequence, the lateral resolution of the obtained image decreases, and the S/N of the acquired image signal also decreases in a confocal optical system. Accordingly, a general conventional approach is to apply a thin beam of about 1 mm which is hardly influenced by the aberration of the eye optical system, and form a spot of about 20 μm on the retina.
The AO technique is beginning to be introduced as a method for avoiding the influence of the aberration of the eye optical system as described above. The following example has been reported so far. That is, even when a thick beam of about 7.5 mm is applied to the eyeball by using this technique, the beam can be condensed to less than 2 μm close to the diffraction limit on the retina by wavefront compensation, and a high-resolution SLO or OCT image is acquired.
Japanese Patent Laid-Open No. 2005-501587 describes an SLO arrangement in which two-dimensional scanning is performed by condensing a light beam from an illumination light source to the retina, and a wavefront detector detects the wavefront by using a portion of return light reflected from the retina. In this arrangement, a wavefront corrector corrects the wavefronts of illumination light and return light, and an image is formed by using the remaining portion of the return light. It is assumed that the DM is used as the wavefront corrector.
Japanese Patent Laid-Open No. 2007-14569 describes an SLO arrangement based on the assumption that a liquid crystal SLM is used as the wavefront corrector. Unlike the example disclosed in Japanese Patent Laid-Open No. 2005-501587 using the DM, illumination light for image acquisition and wavefront detection illuminates the retina without intervening the SLM.
Generally, when a vapor deposition film is optimized, the DM has characteristics independent of the wavelength over a broad wavelength band and hence can be used in a plurality of applications. However, the DM has drawbacks that calculations are complicated, for example, the setting of a pseudo-inverse matrix necessary to calculate a correction value is complicated, and the cost is very high. On the other hand, the liquid crystal SLM has drawbacks that it has the wavelength dispersion characteristic of a liquid crystal material and the dependence of the diffraction efficiency on the wavelength, and can correct only a polarized component in a specific direction. However, the liquid crystal SLM has advantages that it is more inexpensive than the DM, and control is easy because the measured wavefront aberration shape need only be displayed directly.
As for the drawbacks of the liquid crystal SLM, for example, as for the dependence on polarization, the loss of efficiency can be suppressed to some extent by performing control such that the polarization of the illumination light becomes linear polarization parallel to the modulating operation direction of the SLM. As for the dependence on the wavelength, no problem arises when using a light source having a small wavelength width.
A case in which the liquid crystal SLM is used as the wavefront corrector of the AO system as disclosed in Japanese Patent Laid-Open No. 2007-14569 and a Hartmann Shack wavefront sensor combining a microlens array and two-dimensional imaging element is used as the wavefront detector will be described below. In the following description, the microlens array will be referred to as an “MLA”, and the Hartmann Shack wavefront sensor will be referred to as an “HS wavefront sensor”. The wavefront corrector and wavefront detector are generally arranged to be optically conjugated. This is so because wavefront shape data detected by the wavefront detector need only be formed as correction data on the wavefront corrector without being processed. This raises the feedback time response of wavefront correction, and also improves the convergence properties. Accordingly, a modulation pattern displayed on the liquid crystal SLM is projected (imaged) on the MLA of the HS wavefront sensor.
In this state, a maximum phase modulation amount of the liquid crystal SLM is only a little more than 2π, that is, only a little more than one wavelength as an optical path length. To correct an aberration amount of a few wavelengths or more, therefore, a correction control signal value is folded for every phase of 2π (equivalent to one wavelength as an optical path length). Modulation must be performed by thus performing phase wrapping (to be referred to as “PW” hereinafter). Accordingly, a boundary line on which a control signal becomes discontinuous due to PW appears for every phase modulation amount of 2π.
Also, in the HS wavefront sensor, a displacement from a reference position is measured for each spot (a Hartmann Shack image: to be referred to as an “HS image” hereinafter) formed by each micro lens (to be referred to as an “ML” hereinafter) of the MLA. Then, the slope of a wavefront piece between the MLs is calculated from this displacement, and the whole wavefront shape is derived. In this case, if the PW boundary line is projected near the center of each ML, the image formation spot breaks. If the spot shape thus breaks, the spot detection positional accuracy decreases, and this makes it impossible to obtain a correct wavefront.